Moms-to-be who smoke but quit early in pregnancy can sharply reduce their risk of having a premature or too-small baby, new research in the journal Obstetrics & Gynecology shows.

"Our results show that first-trimester quitters have a risk of delivering a preterm or SGA newborn comparable to those who never smoked during pregnancy, and second trimester quitters also have a lower risk of these outcomes, but not to the same magnitude as first-trimester quitters," Dr. Laura L. Polakowski of the Centers for Disease Control and Prevention in Atlanta and her colleagues report.

Smoking during pregnancy has many ill effects, with premature delivery and restricted growth among the best-documented, Polakowski and her team write. For older women, they add, the dangers of smoking in pregnancy may be even greater.

Revised US birth certificates introduced in 2003 include information on whether or not a mother smoked for each trimester of her pregnancy, so the researchers looked at whether quitting at different points in pregnancy might affect women's risk of poor birth outcomes. Their analysis included 915,441 birth certificates from babies born in 2005 in 11 states that used the revised birth certificate format.

Among women who smoked for their entire pregnancy, 10% gave birth to babies preterm but not small for gestational age; 15% had full term babies that were small for gestational age; and 2% had preemies that were also small for gestational age. For women who quit during their first trimester, those risks were 8%, 9%, and 1%, respectively.

After the researchers adjusted for mother's age, previous preterm births, and other relevant factors, they found that the women who quit in the first trimester cut their risk of having a preterm, normal size baby by 31%; a full-term, too-small baby by 55%; and a preterm, small for gestational age baby by 53%.

Women who quit smoking in the second trimester also cut their risk of these outcomes, but not as much. The reduction in risk associated with quitting in the first trimester was particularly strong for older women, especially those over 40, the researchers found.

"Although pregnancy is the most common reason women quit smoking, smoking is a difficult addiction to break," Polakowski and her colleagues write. "Added incentive to quit may come with further evidence of the benefits of smoking cessation even as pregnancy progresses."

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